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Useful Notes / Tyrannosaurus rex

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"Sue" on display at the Field Museum of Natural History in Chicago, Illinois. Photographed by Evolutionnumber9 and licensed under CC BY-SA 4.0.
There's no doubt that Tyrannosaurus rex is, by far, the most famous of the Stock Dinosaurs, seen as both the most majestic and most terrifying of them. However, there are lots of common misconceptions about the animal, which we attempt to clear up here.

  • Name: Whereas most dinosaurs are known outside paleontological circles only by their genus, T. rex is known by their full binomium. Tyrannosaurus, the genus name, comes from the Greek tyrannos, meaning "tyrant", and sauros, meaning "lizard". Rex, the specific name, meanwhile, is Latin for "king", therefore the whole name translates to "tyrant lizard king". As per the rules of binomial nomenclature, the genus name should be spelled with a capital T, whereas the species name should be spelled with a lowercase r. And the correct abbreviation of the name is T. rex, not T-rex and especially not T-Rex. Early fossils were described under the names Dynamosaurus imperiosus and Manospondylus gigas — both of them are now considered invalid synonyms. A number of other names have also become invalid synonyms of Tyrannosaurus over the years, including Stygivenator and Alamotyrannus. One particular synonym, Nanotyrannus, was long believed to be a dwarf T. rex relative for its slender frame and narrower snout before scientists realized that juvenile T. rex simply lacked the heavy skulls and robust bodies of the grown-ups. As of 2022, it's been suggested that the oldest specimens of Tyrannosaurus, from about 68 million years ago, belonged to a separate species called T. imperator ("tyrant lizard emperor") that then evolved into T. rex and another, more gracile species called T. regina ("tyrant lizard queen"), but this is not widely accepted. T. imperator would be the species whose famous giant specimen named "Sue" belongs to.
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  • Discovery: The first T. rex fossils were found in the late 1800s and were believed to belong to a giant ornithomimid or a ceratopsid. The species was officially described by Henry Farfield Osborn in 1905, based a partial skeleton consisting of 34 bones, found by curator Barnum Brown in Hell Creek, Montana. Osborn described another fossil of a large carnivore, found in Wyoming, as Dynamosaurus imperiosus ("ruling power lizard"), but then realized that the two belonged to the same species. The most complete T. rex skeletons were found in 1990 and 1992, and were dubbed as "Sue" and "Stan". These two fossils helped us get a much more accurate image of what the species was like.
  • Classification: It was once believed that Tyrannosaurus was closely related to other giant carnivorous dinosaurs, such as Allosaurus and Giganotosaurus, as part of a group of dinosaurs called Carnosauria. However, it's now agreed upon that T. rex was actually part of the group Coelurosauria, which includes many small, feathered dinosaurs, such as Velociraptor and Archaeopteryx, as well as modern birds. This means T. rex's ancestors were likely quite similar in appearance. Within Coelurosauria, T. rex was part of the family Tyrannosauridae. Other tyrannosaurs include Tarbosaurus (once sometimes considered the Asian Tyrannosaurus species), Albertosaurus, Gorgosaurus, Daspletosaurus, Nanuqsaurus, and Alioramus. They were part of the superfamily Tyrannosauroidea, which also includes Alectrosaurus, Dryptosaurus, Yutyrannus, Proceratosaurus, Eotyrannus, Dilong, Guanlong, and possibly Megaraptor.
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  • Time period: T. rex lived at the very end of the Cretaceous period, 68-66 million years ago (also known as the Maastrichtian age). It was among the few dinosaurs that was still around when the famous asteroid collision ended the Mesozoic era (others included Triceratops, Ankylosaurus and Edmontosaurus). Any depiction of T. rex earlier than that is, therefore, inaccurate.
  • Range: T. rex was an exclusively North American species. During the Late Cretaceous, North America was divided to two smaller continents by a shallow sea named the Western Interior Seaway; T. rex lived on the Western continent, dubbed Laramidia. It was found throughout Laramidia, ranging from Alberta in the north to Texas and New Mexico in the south. A closely related species, the aforementioned but smaller Tarbosaurus bataar, lived in East Asia at the same time.
  • Size: T. rex was famous for being one of the largest carnivorous dinosaurs. The "Sue" skeleton is 12.3 to 12.8 m long (the inaccuracy comes from a few missing vertebrae), about 4 m tall at the hip, and is estimated to weigh 8.4 to 14 metric tons (for comparison, that's 2 to 3 times the weight of an African bush elephant). It was also much bulkier in real life than the athletic, "cut"note  manner in which fiction tends to depict it. As a result, it probably wasn't very fast, but given that most of its usual prey was also large and bulky, it didn't have to be.
  • Posture: Early restorations depicted T. rex in a kangaroo-like tripod posture, dragging its long tail on the ground. However, thanks to more complete skeletons, now we know that it held its body horizontally, balancing its body with its tail.
  • Big head: T. rex had a massive head even in comparison to other carnivorous dinosaurs. Its mouth was full of sharp teeth, up to 20 cm (8 inches) long, sometimes dubbed "killer bananas" because of their size and shape. Its bite force is estimated to be about 8,000 pounds, stronger than any other known land animal, which it needed to crush the bones of the large, often armored dinosaurs it ate.
  • Puny arms: One of the most iconic, and most ridiculed, traits of T. rex is its tiny arms. It was two-fingered, with sharp claws on them, and its palms faced inward (rather than downward, as it's often erroneously depicted). The reason for the arms' small size is mainly practicality; large arms would have gotten in the way of the Tyrannosaurus's bite, which is theorized to have been the most powerful bite of all dinosaurs, hence the need to have small arms where other predators would have clawing weapons. Additionally, in spite of their size the arm bones show signs of large muscle attachment and thus, they were very strong and capable of lifting 200 pounds (90 kilograms). Because of this, T. rex might have used them to hold onto struggling prey while it dispatched it with its jaws. In addition, they could have also been used to help lift the T. rex from a sleeping position when it was waking up. Interestingly, baby and juvenile Tyrannosaurus had slightly larger arms compared to their total body size.
  • Hunter or scavenger: Though the T. rex is typically portrayed as a hunter in media, there was an infamous debate among paleontologists as to whether or not T. rex was actually a scavenger instead, popularized by paleontologist and Jurassic Park dinosaur consultant Jack Horner. While the idea of a large, bulky carnivore solely surviving on rotting carrion is just too ridiculous to be taken seriously (due to the amount of food needed to sustain its massive size), it is theorized that large, adult Tyrannosaurus would hunt less and basically Kill Steal the hunts of other carnivores and younger T. rex; the highly advanced olfactory sense of the creature allowed it to sniff out carrion from miles away, and a roar would have definitely helped scare off smaller dinosaurs from a kill. Due to an adult's size and strength, however, it's also very possible that it simply fought heavily armored animals like Triceratops or Ankylosaurus and won, while also being able to overpower 12-meter behemoths like Edmontosaurus. On the other hand, younger, smaller Tyrannosaurus, with their more slender bodies and narrower snouts, hunted faster-moving and more delicate mid-sized animals like Ornithomimus, Thescelosaurus, and Pachycephalosaurus. Even in that case, a roar could've been helpful; even the most active predators are unlikely to pass up the opportunity to steal a free meal if they can, and a Tyrannosaurus might well have needed to threaten off other dinos (including other Tyrannosaurus) while eating. Bite marks on T. rex bones attributed to other T. rexes also suggest it may have even been cannibalistic.
  • Feathers or scales: Historically, T. rex was portrayed with lizard- or crocodile-like scaly skin. As many dinosaur species were discovered to be covered in feathers, including a tyrannosaur relative, Yutyrannus, it was suggested that T. rex could also have been feathered, basically looking like a giant toothy bird. However, fossilized skin impressions of T. rex and other, closely related tyrannosaurs show that most of its body was, indeed, scaly; the only place that was potentially feathered is its back, similar to the mane of a lion. As T. rex was a large animal living in warm climate, it likely did not need the extra insulation from feathers, just as similarly-sized mammals like rhinoceroses and elephants are sparsely haired. Some still speculate that it had downy feathers as a hatchling, when it was still small enough to need insulation, and eventually lost these as it reached a certain age and size, much like a baby penguin molting away its birth feathers as it becomes an adolescent.
  • Sound: T. rex is typically depicted in media with a Mighty Roar; ever since Jurassic Park, everyone "knows" what that roar sounded like. However, there is actually little evidence that T. rex could roar; it is speculated to have produced low-pitched, rumbling or bellowing sounds, similar to crocodilians or large flightless birds (which would've still sounded pretty damned impressive, but possibly a bit underwhelming for those used to the movies). The vibrations produced by a communicating T. rex may have even been deep and powerful enough to have been felt when they rippled through the air as well as creating small earthquakes.
  • Senses: T. rex had extraordinary senses of smell and hearing. Analysis of the braincase in fossilized skulls shows that it had large olfactory bulbs and a long cochlear duct capable of receiving low-frequency sounds. These traits would have been advantageous both as a predator (hunting prey) and as a scavenger (finding carrion from a great distance or detecting approaching rivals). However, contrary to what Steven Spielberg and Michael Crichton would have you believe, T. rex also had excellent eyesight that would've let the creature spot objects from as far as six kilometres with thirteen times the detail. It had large eyes facing forward connecting to big optical lobes, and a relatively narrow snout, allowing for binocular vision. This ability supports the idea that T. rex was a hunter, as binocular vision is beneficial when chasing prey.
  • Intelligence: Most early depictions of T. rex depict them as solitary brutes with little in the way or brainpower or social ability. More recent studies have shown that T. rex had a bigger brain-to-body ratio than previously thought, being smarter than earlier species of giant theropods. Given its adaptations and how its common prey items included some heavily armored and highly social animals, its likely T. rex would need the intelligence to be able take down its prey with different strategies and know when to throw in the towel and eat a pre-killed carcass instead. Some scientists have also proposed that T. rex sometimes hunted in packs, but such coordinated behavior in predatory dinosaurs is hotly debated.